<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    function num(){
      let p = Number(prompt('请输入银行卡余额'))
      let o = Number(prompt('请输入当月食宿消费金额'))
      let b = Number(prompt('请输入当月生活消费金额'))
      let m = p - o - b
      document.write(`您的银行卡余额为${m}`);
      return m
    }
    let i = num()
    console.log(i);
  </script>
</body>
</html>